Trouble with IFrame in MS CRM 4.0

I have created an IFrame in a form. It shows the website (internal site) fine but when I type a username and password to login to the site it opens the site in a new window. How do I keep this from happening?

Let me back up a little. Attached is an image that shows where the web address that I want to show up in the IFrame is.

I cannot add the code you provided to the website. It is a 3rd party site created for us. Because each application that is opened has a different web address I need the IFrame to use the address that is in the field.

In that case I assume that site is coming from a third party server as well so most probably you will not be able to modify it via JavaScript.
Would you like to give it a shot though? Programatically add <base href="_self"/> tag?

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The site is on a server here in our office. I physically cannot add it.

Here is a tutorial that I found through google. Maybe the information is bad.

To do this, one sets up a little code for the form On Load event that puts the Accounts web site URL into the IFRAME’s URL. It’s actually pretty simple. Here is the code (assumes you already know how to set up an IFRAME). IFRAME_WebSite is the name of the IFRAME and .src references the URL it uses.

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